Chromium-free rust-inhibitive surface treatment agent for metal parts with zinc surfaces and metal parts with zinc surfaces coated with rust-inhibitive surface coated film

ABSTRACT

Disclosed is a chromium-free rust-inhibitive surface treatment agent to form a siliceous film that rarely cracks or peels off and yields an excellent rust-inhibitive performance on zinc surfaces of a metal part. The chromium-free rust-inhibitive surface treatment agent is an alcoholic solution of alkoxysilane oligomer having weight-averaged molecular weight of 1,000 to 10,000, and 2.5 to 15% of silicon in molecules of the alkoxysilane oligomer has been replaced with titanium. To prepare partly titanium-replaced alkoxysilane oligomer, titanium compound, in which about a half of alkoxy groups in titanium tetraalkoxide has been chelated, is reacted with tetraalkoxysilane monomer or alkoxysilane oligomer in the alcoholic solution.

This is a divisional of application Ser. No. 12/828,979 filed Sep. 15,2008, which is a 371 of PCT/JP2007/058137 filed Apr. 13, 2007, whichclaims priority from JP 2006-114099 filed Apr. 18, 2006; the contents ofall of which are incorporated herewith by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a chromium-free rust-inhibitive surfacetreatment agent used for preventing the generation of white rust and redrust on metal parts with zinc surfaces such as galvanized bolts andnuts, and to metal parts with zinc surfaces coated with arust-inhibitive surface coated film.

BACKGROUND ART

RoHS regulation has come into effect in the Europe, and the use ofsurface treatment agents containing a hexavalent chromium component suchas chromate treatment (surface treatment using hexavalent chromiumcomponents) provided for imparting rust inhibitive performance on zincsurfaces has been restricted. Along with the regulation, chromitesurface treatment performed for galvanized metal parts has becomewidespread instead of chromate treatment in industrial fields. However,the chromite treatment has some problems. For example, there areproblems that it is difficult to control treatment solutions; therenewal life of treatment solutions is short; and the frictioncoefficient on the surface of galvanized metal parts subjected tochromite treatment is large. It is necessary to further apply a surfacetreatment agent for adjusting a friction coefficient in fastener partssuch as some galvanized bolts and nuts. In addition, the rust inhibitiveperformance of galvanized products subjected to chromite surfacetreatment is inferior to that of galvanized products subjected toconventional chromate treatment, and sometimes relaxation of the rustinhibitive specification required for galvanized parts has also beenconducted.

Further, it cannot be prevented that some trivalent chromium isconverted into hexavalent chromium by an equilibrium reaction andsignificant amounts of hexavalent chromium components are detected inthe conversion film. For this reason, it is considered that the chromitesurface treatment method is a temporary surface treatment method andshould be changed to a completely chromium-free surface treatment methodin the near future.

Many treatment methods have so far been proposed for completelychromium-free surface treatment. However, in galvanized metal partsprovided with a thin coated film obtained by chromium-free surfacetreatment which is equivalent in thickness to that obtained by chromatetreatment, the rust-inhibitive performance has not yet reached thepractical use. In the case of applying top coating, the surfaces ofgalvanized metal parts should be subjected to chromium-free surfacetreatment with a film having a thickness exceeding 10 μm to achieve therequired rust-inhibitive performance. However, there is no chromium-freerust-inhibitive surface treatment only with thin coated films forgalvanized metal parts showing rust-inhibitive performance which is notinferior to that obtained by chromate treatment, except for thechromium-free rust-inhibitive surface treatment which the presentinventors have previously proposed.

In Patent Document 1, the present inventors have proposed achromium-free rust-inhibitive surface treatment agent which is appliedonto the surfaces of galvanized metal parts to form a thin siliceousfilm which can suppress the generation of red rust for a long period oftime. Nano-sized powders of titanium oxide which are subjected todispersion treatment and have an averaged primary particle size of 70 nmor less have been blended in an effective amount in the chromium-freerust-inhibitive surface treatment agent.

Further, in Patent Document 2, the present inventors have proposed achromium-free rust-inhibitive surface treatment agent for zinc surfacescomposed mainly of an alcoholic solution of alkoxysilane oligomer havinga specific weight-averaged molecular weight. When the chromium-freerust-inhibitive surface treatment agent is applied onto the zincsurfaces of galvanized products and the like to form a thin siliceousfilm, the generation of white rust can be suppressed for a long periodof time.

In utilizing the chromium-free rust-inhibitive surface treatment agentcomposed mainly of an alcoholic solution of alkoxysilane oligomer, whengalvanized metal parts are subjected to an activation treatment in whichthe metal parts are immersed in a diluted aqueous nitric acid solution(a pickling process performed as pre-treatment of chromate treatment)and then subjected to chromium-free surface treatment, therust-inhibitive performance is poor in many cases. For this reason,galvanized metal parts not subjected to nitric acid activation treatment(pickling) are water-rinsed and dried, and then they are subjected tochromium-free rust-inhibitive surface treatment.

Further, when the chromium-free rust-inhibitive surface treatment agentcomposed mainly of an alcoholic solution of alkoxysilane oligomer isapplied onto as-galvanized bolts (without chromate treatment) obtainedfrom several galvanizers, there is a problem that the rust-inhibitiveperformance in the generation of white rust is greatly changed dependingon which galvanized bolt is applied.

As a method which can prevent this problem, in Patent Document 3, thepresent inventors have proposed rust-inhibitive treatment in which metalparts are previously subjected to chromium-free conversion coating andthen applied with a chromium-free rust-inhibitive surface treatmentagent composed mainly of an alcoholic solution of alkoxysilane oligomer.However, the surface treatment needs to add at least one treatmentprocess and therefore cannot satisfy the request of users who wish toperform surface treatment in a simple process.

Thereafter, when galvanized metal parts such as bolts are applied withthis chromium-free rust-inhibitive surface treatment agent and thecoated metal parts are stored for about 1 year, there arises a problemthat: crazing occurs in the coated film; a phenomenon that crazingoccurs in the coated film and subsequently the film is peeled off insome portion where a little thick coated film (3 μm or more) is applied;and white powders are observed on the galvanized surface and it seems asif white rusting is generated.

Patent Document 4 has not mentioned rust-inhibitive performance, but hasdisclosed a coating composition in which an alkoxysilane is hydrolyzedby adding an acid catalyst and water and condensation-polymerized whileevaporating alcohol and water. The every alkoxysilane used as a rawmaterial of the coating composition in Examples is an alkylalkoxysilane.In addition, it is described that a chelate compound of zirconium,titanium or aluminum is blended into this composition. However, there isan example in which a zirconium chelate compound is blended (see Example7 in the same Patent Document 4), but there is no example in which anorganic chelate titanium compound is blended or there is no example inwhich metal parts with zinc surfaces are coated.

Further, Patent Document 5 has disclosed a silica-based protectioncoating solution in which an alkoxysilane and a titanium alkoxide arehydrolyzed in an alcoholic solution using acetic acid as a catalyst andthen condensation-polymerized. In the examples, examples of an object tobe protection-coated include CFRP (carbon fiber reinforced plastics) inaddition to metal parts such as titanium. In addition, as thealkoxysilane used as a raw material of a protection coating solution, analkoxysilane having an epoxy functional group and an alkoxysilane havingan amino group (exhibiting basicity) are used. Since an amino groupfunctions as a basic catalyst for proceeding with a cross-linkingreaction between alkoxysilane oligomer molecules in the solution, thereis a disadvantage that the solution is apt to gelated. In Example 4 inthe same Patent Document 5, after the protection coating solution isapplied onto a hot-dip galvanized steel plate using a mixed coatingsolution of alcohol and water, the adhesion of the coated film to thesubstrate is evaluated, but the rust-inhibitive performance is notexamined.

-   Patent Document 1: JP 2005-097719 A-   Patent Document 2: JP 2005-264170 A-   Patent Document 3: JP 2006-225761 A-   Patent Document 4: JP 07-157715 A-   Patent Document 5: JP 2003-160759 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

There are some problems in the previously proposed chromium-freerust-inhibitive surface treatment agent composed mainly of an alcoholicsolution of alkoxysilane oligomer. Particularly, when metal partsgalvanized in various different conditions are subjected to surfacetreatment with the previously proposed chromium-free rust-inhibitivesurface treatment agent, the satisfactory rust-inhibitive performancecannot be obtained in some cases. Further, as moisture in the air isincorporated into the solution, a small amount of dispersion-treatednano-sized powders of titanium oxide which is blended for enhancingrust-inhibitive performance tends to be agglomerated. When a surfacetreatment agent containing the agglomerated nano-sized powders oftitanium oxide is applied onto a zinc surface of a metal part to form acoated film, the surface becomes a little whitish, and it seems as ifwhite rust is generated. In addition, as a phenomenon intrinsic to acoated film formed by a sol-gel method, when the applied coated film isbaked after drying, tensile stress is generated in the resulting coatedfilm along with evaporation of a solvent, and thereby crazing occurs inthe coated film by this stress. The coated film tends to be peeled offin the portion where crazing occurs with the lapse of time if a littlethick portion is present in the coated film. When the coated film ispeeled off, it forms white powders, and it seems as if white rust isgenerated on the surface of the metal part, and thereby therust-inhibitive performance in the portion is reduced.

The present invention has an object to provide a chromium-freerust-inhibitive surface treatment agent which resolves problems of thepreviously proposed chromium-free rust-inhibitive surface treatmentagent composed mainly of an alcoholic solution of alkoxysilane oligomerto improve rust-inhibitive performance.

That is, the object of the present invention is to provide achromium-free rust-inhibitive surface treatment agent having furtherimproved rust-inhibitive performance which is able to impartrust-inhibitive performance for practical use to even galvanized partsthat is formerly difficult to impart rust-inhibitive performance forpractical use and has a poor affinity with a surface treatment agent;and in which crazing or peeling off hardly occurs to the formedrust-inhibitive coated film.

Means for Solving the Problems

A chromium-free rust-inhibitive surface treatment agent for metal partswith zinc surfaces of the present invention consists of an alcoholicsolution of alkoxysilane oligomer having weight-averaged molecularweight (Mw) of 1,000 to 10,000, wherein silicon atoms in molecules ofthe alkoxysilane oligomer are partly replaced with titanium atoms froman organic chelate titanium compound, the alcoholic solution containstitanium of 2.5 to 15 atomic % to a total amount of silicon andtitanium, and the total amount of silicon and titanium is 5 to 20 weight% in the alcoholic solution when silicon and titanium are converted toSiO₂ and TiO₂, respectively.

The alcoholic solution of alkoxysilane oligomer is preferablysynthesized by adding an acid catalyst and water to an alcoholicsolution containing alkoxysilane raw material and the organic chelatetitanium compound, and hydrolyzing and condensation-polymerizing thealkoxysilane raw material and the organic chelate titanium compound.

Alternatively, the alcoholic solution of alkoxysilane oligomer may beprepared by adding an acid catalyst and water to an alcoholic solutioncontaining an alkoxysilane raw material to hydrolyze andcondensation-polymerize the alkoxysilane raw material to synthesizealkoxysilane oligomer, and mixing an organic chelate titanium compoundwith an alcoholic solution of the synthesized alkoxysilane oligomer.

In the present invention, the organic chelate titanium compound ispreferably titanium alkoxide in which 40 to 60% of alkoxy groups areblocked or replaced by a chelate agent.

In the present invention, the alkoxysilane raw material used for thesynthesis of alkoxysilane oligomer consists of 90 to 99 mol % oftetraalkoxysilane monomer or low molecular weight tetraalkoxysilanesource oligomer (which have weight-averaged molecular weight (Mw) lessthan 800, and in a case of using low molecular weight oligomer, the mol% is determined by the total molar amount of the polymerized monomers),and the balance being alkylalkoxysilane monomer.

In the present invention, the alkylalkoxysilane monomer is preferably atleast one selected from the group consisting of methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, vinyltrimethoxysilane andγ-methacryloxypropyltrimethoxysilane.

In the present invention, the chelate agent is preferably β-diketone oroctylene glycol. As β-diketone, acetylacetone is preferably used.

The alcoholic solution of alkoxysilane oligomer of the present inventionpreferably contains 0.1 to 2 weight % of alcohol-soluble resin. Thealcohol-soluble resin is preferably polyvinyl butyral.

The alcoholic solution of alkoxysilane oligomer of the present inventionpreferably contains 0.004 to 0.10 weight % of boric acid.

In the present invention, 20 to 40 weight % of alcohol component in thealcoholic solution of alkoxysilane oligomer is preferably alcohol orglycol ether having a boiling point of 97° C. or more.

Further, the alcohol or the glycol ether having a boiling point of 97°C. or more is preferably at least one selected from the group consistingof n-propyl alcohol (boiling point: 97° C.), n-butyl alcohol (boilingpoint: 117° C.), propylene glycol monomethyl ether (hereinafter,abbreviated as “PGME”, boiling point: 121° C.), ethylene glycolmonoethyl ether (sometimes referred to as “ethyl cellosolve”, boilingpoint: 136° C.) and ethylene glycol tert-butyl ether (hereinafter,abbreviated as “ETB”, boiling point: 152.5° C.).

The metal part with zinc surfaces of the present invention is coatedwith a siliceous film of 0.5 to 3 μm in average thickness formed by thechromium-free rust-inhibitive surface treatment agent.

The zinc surface of a metal part is preferably coated with thechromium-free rust-inhibitive surface treatment agent by a dip and spincoating method to form a siliceous film thereon. In addition, thesiliceous film is preferably baked at a temperature of 120° C. or lessafter coating.

In the present invention, a “film containing silica as a maincomponent”, that is, a “siliceous film” means a film containing 65weight % or more of SiO₂ component.

Advantage of the Invention

In the chromium-free rust-inhibitive surface treatment agent of thepresent invention, silicon atoms in the alkoxysilane oligomer preparedas an alcoholic solution are partly replaced with titanium atoms, andthereby the rust-inhibitive performance of the zinc surface coated witha siliceous film can be improved, the time until the generation of whiterust is extended to 210 hours or more and the time until the generationof red rust is extended to 1150 hours or more in a salt spray test, andsimultaneously the adhesion of the siliceous film to the zinc surfacecan be improved.

When the alcoholic solution of the chromium-free rust-inhibitive surfacetreatment agent according to the present invention is applied onto thezinc surface of a metal part to form a thin siliceous film of 0.5 to 3μm, white rusting resistance superior to that provided by conventionalchromate treatment or the chromium-free rust-inhibitive surfacetreatment agent composed mainly of an alcoholic solution of alkoxysilaneoligomer which the present inventors have previously proposed can beimparted to the zinc surface of a metal part. Further, a crazingphenomenon that occurs in the siliceous film with a lapse of time can besuppressed, and a phenomenon that the crazed siliceous film is peeledoff from the zinc surface can be prevented.

Even when the siliceous film formed on the zinc surface of a metal partby the chromium-free rust-inhibitive surface treatment agent of thepresent invention is damaged with a knife or the like, the filmcomponent is diffused to the damaged portion in moist air to cover thedamaged portion with a thin film, thereby exhibiting self-repairingproperties for preventing the generation of white rust.

BEST MODE FOR CARRYING OUT THE INVENTION

The alkoxysilane oligomer which is a main effective component of thechromium-free rust-inhibitive surface treatment agent according to thepresent invention is an alkoxysilane oligomer in which silicon atoms inthe molecule of the oligomer are partly replaced with titanium atoms,and has a molecular structure where oxygen and silicon or titanium arealternately bonded and forms a linear molecule with the lengthexhibiting good film forming property.

When the weight-averaged molecular weight (Mw) of the alkoxysilaneoligomer molecule is too small, the film forming property and therust-inhibitive performance capable of imparting to metal parts becomepoor, when the Mw is too large, the stability of the alcoholic solution(which means storage stability and becomes unusable due to occurrence ofgelation with a lapse of time) is impaired and therefore the molecule ofalkoxysilane oligomer needs to have weight-averaged molecular weight(Mw) of 1,000 to 10,000.

The alkoxysilane oligomer obtained by adding an acid catalyst and waterto an alcoholic solution of alkoxysilane raw material to hydrolyze andcondensation-polymerize the resulting mixture is composed of linearmolecules, and the linear molecules are supposed to be either a singlelinear molecule or a ladder-type linear molecule. On the other hand, thealkoxysilane oligomer obtained by condensation-polymerizing using abasic catalyst in the alcoholic solution of alkoxysilane raw materialtends to proceed with three-dimensional condensation-polymerization ofthe oligomer and is easily gelled, and therefore the alkoxysilaneoligomer solution exhibits poor storage stability.

The alkoxysilane oligomer in which silicon atoms in the molecule arepartly replaced with titanium atoms has more preferably weight-averagedmolecular weight (Mw) of 1,500 to 5,000. In the present invention, theweight-averaged molecular weight (Mw) of the alkoxysilane oligomer canbe measured using polystyrene standards and a tetrahydrofuran solvent bya gel permeation chromatography.

A siliceous film is formed on the zinc surface of a metal part using thechromium-free rust-inhibitive surface treatment agent consisting of analcoholic solution of alkoxysilane oligomer in which silicon atoms inthe molecule are partly replaced with titanium atoms, thereby exhibitinggood rust-inhibitive performance, and even the siliceous film thinnerthan 1 μm can impart rust-inhibitive performance for practical use to ametal part with zinc surfaces. It is effective to make a siliceous filmthinner for preventing crazing in the film and peeling off of the filmformed.

Further, the siliceous film formed by applying an alcoholic solution ofthe alkoxysilane oligomer in which silicon atoms in the oligomer arepartly replaced with titanium atoms on the zinc surface of a metal parthas excellent adhesion to the zinc surface of a metal part, and evenwhen crazing occurs in the siliceous film, the siliceous film does notpeel off from the zinc surface.

When the replacement ratio of silicon atoms with titanium atoms is smallin the molecule of alkoxysilane oligomer, the rust-inhibitiveperformance imparted to metal parts with zinc surfaces by the siliceousfilm formed by the chromium-free rust-inhibitive surface treatment agentis poor, and when the replacement ratio is too large, the cost becomestoo high from the viewpoint of attained rust-inhibitive performance, andtherefore the replacement ratio is preferably 2.5 to 15 atomic %. Thereplacement ratio of silicon atoms in the molecule of alkoxysilaneoligomer with titanium atoms is more preferably 3 to 10 atomic %.

The alkoxysilane oligomer in which silicon atoms are partly replacedwith titanium atoms can be synthesized by adding a small amount of anacid catalyst such as hydrochloric acid and water to an alcoholicsolution containing an alkoxysilane raw material and a titaniumalkoxide, and hydrolyzing and condensation-polymerizing the alkoxidemixture. However, the rapid hydrolysis of titanium alkoxide having highactivity generates precipitation, and therefore it is preferred that 40to 60% of alkoxy groups of titanium alkoxide are blocked or replaced bya chelate agent to decrease the reaction activity of titanium alkoxidebefore titanium alkoxide is mixed and condensation-polymerized with analkoxysilane raw material in an alcohol solvent.

As the titanium alkoxide, titanium tetraalkoxide is preferably used. Asthe titanium tetraalkoxide, titanium tetraisopropoxide and titaniumtetra-n-butoxide can be used. As the chelate agent for blocking orreplacing alkoxy groups, β-diketone such as acetylacetone or octyleneglycol can be used. Acetylacetone reacts with the zinc surface toconsume the zinc layer, and therefore octylene glycol having lowreaction activity with the zinc surface is preferably used as a chelateagent for titanium alkoxide in the chromium-free rust-inhibitive surfacetreatment agent of the present invention.

For the chromium-free rust-inhibitive surface treatment agent of thepresent invention, alkoxysilane monomer or low molecular weight oligomer(which have weight-averaged molecular weight (Mw) less than 800, and themol % in a case of utilizing the oligomer is determined by the totalmolar amount of the polymerized monomers) is used as an alkoxysilane rawmaterial, and an acid catalyst such as hydrochloric acid is added to thealcoholic solution of monomer or oligomer to hydrolyze andcondensation-polymerize the alkoxysilane raw material, and thereby analkoxysilane oligomer of a linear molecule having a desiredweight-averaged molecular weight is prepared. As the acid catalyst, inaddition to mineral acids such as hydrochloric acid, sulfuric acid andnitric acid, organic acids such as acetic acid can be used.

Alternatively, a chromium-free rust-inhibitive surface treatment agentcan also be prepared by previously making an alcoholic solution ofalkoxysilane oligomer which is condensation-polymerized using an acidcatalyst so as to have a desired weight-averaged molecular weight, andthen mixing an alcoholic solution of an organic chelate titaniumcompound in this solution to react the organic chelate titanium compoundwith the alkoxysilane oligomer.

When the alcoholic solution of the organic chelate titanium compound andthe alcoholic solution of alkoxysilane oligomer having a desiredweight-averaged molecular weight (Mw) are mixed afterwards, analkoxysilane oligomer to which the molecule of the organic chelatetitanium compound is additionally polymerized can be obtained.

The solution obtained by mixing an alcoholic solution of titaniumalkoxide chelated with octylene glycol with an alcoholic solution ofalkoxysilane monomer or alkoxysilane oligomer has less yellow coloringas compared with the solution obtained by mixing an alcoholic solutionof titanium alkoxide chelated with acetylacetone with an alcoholicsolution of alkoxysilane oligomer.

In the chromium-free rust-inhibitive surface treatment agent of thepresent invention, in order to form a siliceous film with a thicknessproviding rust-inhibitive performance for practical use on the zincsurface of a metal part, the total amount of silicon and titanium asconverted to SiO₂ and TiO₂, respectively, is 5 to 20 weight % in analcoholic solution of alkoxysilane oligomer containing a titaniumcomponent. The total amount of silicon and titanium is more preferably 7to 15 weight %.

The alkoxysilane oligomer of a higher weight-averaged molecular weight(Mw) can form a siliceous film having excellent rust-inhibitiveperformance, and crazing hardly occurs in the film, and therefore the Mwis preferably as high as possible in such an extent that storagestability of the rust-inhibitive surface treatment agent solution is notimpaired. Specifically, an alkoxysilane oligomer iscondensation-polymerized so as to have weight-averaged molecular weight(Mw) of 1,000 to 10,000, and preferably 1,500 to 5,000.

Such an alkoxysilane oligomer can be synthesized by adjusting the pH ofthe alcoholic solution of mixed starting materials to about 4. Synthesisof alkoxysilane oligomer is preferably performed while maintaining thetemperature of the alcoholic solution at 35° C. to 45° C. so that theprogress of condensation-polymerization reaction becomes a saturatedstate in a short period of time. If the condensation-polymerizationreaction is saturated, the progress of the reaction is slow while thealcoholic solution of alkoxysilane oligomer is kept at room temperature,and the alcoholic solution of alkoxysilane oligomer has good storagestability.

An organic component can be introduced into an alkoxysilane oligomer bycopolymerizing a silane coupling agent that is an alkylalkoxysilanemonomer having an organic group such as an alkyl group withtetraalkoxysilane monomer or low molecular weight alkoxysilane oligomer.In addition, an organic component can be introduced into a siliceousfilm by dissolving an alcohol-soluble organic resin in an alcoholicsolution of alkoxysilane oligomer.

As the alkoxysilane raw material, tetraethoxysilane, tetramethoxysilane,and low molecular weight alkoxysilane oligomer obtained bycondensation-polymerizing these monomers which are low in cost arepreferably used. As the low molecular weight alkoxysilane oligomer,commercially available ethyl silicate 40 (pentamer molecule, Mw≈745) ormethyl silicate 51 (tetramer molecule, Mw≈470) can be used.

Further, the chelate agent used for suppressing the reaction activity oftitanium alkoxide remains in the alcoholic solution of alkoxysilaneoligomer after synthesized, and even after a coated film of arust-inhibitive surface treatment agent is baked on the zinc surface ata temperature of about 100° C., the chelate agent remains in thesiliceous film and is supposed to be partly responsible for softening ofthe formed siliceous film.

When a chelate agent of more than 2 mols is blended with 1 mol oftitanium alkoxide, the excess of the chelate agent over 2 mols is notconsumed for formation of an organic chelate titanium compound due tothe steric hindrance. As the chelate agent used for titanium alkoxide,β-diketone such as acetylacetone or octylene glycol is preferably used.

When acetylacetone is excessively blended, it is contained in analcoholic solution as a high-boiling point solvent becauseacethylacetone has a boiling point as high as 140° C. Since octyleneglycol is a high-boiling point solvent having a boiling point over 240°C., it functions as a high-boiling point solvent in the alcoholicsolution similarly to acetylacetone.

When an alkylalkoxysilane monomer having organic groups directly bondedto the silicon atoms is partly introduced into an alkoxysilane oligomer,an alkoxysilane oligomer having organic groups bonded to the siliconatoms is formed. The siliceous film formed on the metal part surface canbe softened and the effect of suppressing craze in the film can beobtained. The alkylalkoxysilane monomer is preferably at least oneselected from the group consisting of methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, vinyltrimethoxysilane andγ-methacryloxypropyltrimethoxysilane which have a low tendency todeteriorate rust-inhibitive performance. Particularly,vinyltrimethoxysilane is a preferred alkylalkoxysilane monomer.

However, when organic groups are excessively introduced, therust-inhibitive performance obtained when a siliceous film is coated onthe zinc surface tends to be reduced, and therefore an alkylalkoxysilanemonomer is blended in a ratio of preferably 1 to 10 mol %, morepreferably 2 to 8 mol % based on the total of alkoxysilane monomer inthe raw materials to synthesize an alkoxysilane oligomer.

Further, a resin component can be introduced into a formed siliceousfilm by dissolving an alcohol-soluble resin in an alcoholic solution ofa chromium-free rust-inhibitive surface treatment agent, and applyingthe rust-inhibitive surface treatment agent onto the zinc surface of ametal part. When the resin component is introduced into the siliceousfilm, the film becomes softened, and thereby the occurrence of crazingin the film can be suppressed. When the alcohol-soluble organic resin iswater-soluble, the water-resistance of the siliceous film formed isimpaired, and therefore a resin which is soluble in alcohol butinsoluble in water is preferably selected.

As the resin suitable for this purpose, a polyvinyl butyral resin ispreferred. The amount of polyvinyl butyral resin dissolved in analcoholic solution of a chromium-free rust-inhibitive surface treatmentagent is preferably 0.1 to 2 weight % and more preferably 0.2 to 1weight %.

When a small amount of boric acid, in addition to organic resins such asa polyvinyl butyral resin, is dissolved in the alcoholic solution of thechromium-free rust-inhibitive surface treatment agent, the effect ofsuppressing the occurrence of craze is obtained. 0.004 to 0.10 weight %of boric acid is preferably dissolved in the alcoholic solution.

A dip and spin coating method is a coating method in which metal partswith zinc surfaces are immersed in an alcoholic solution of achromium-free rust-inhibitive surface treatment agent; the wet metalparts are put out from the alcoholic solution to place them in a metalbasket mounted on a centrifugal separator; and the basket is rotated toswing away an excess amount of the alcoholic solution of thechromium-free rust-inhibitive surface treatment agent adhered to themetal part surfaces by the centrifugal force. The metal parts coatedwith a thin liquid film (coated film) taken out from the basket aredried and baked at a temperature of about 100° C. to form a siliceousfilm.

The thickness of a coated film of a rust-inhibitive surface treatmentagent formed on a zinc surface of metal parts by a dip and spin coatingmethod is affected by the concentration of alkoxysilane oligomer in thealcoholic solution and the viscosity of the alcoholic solution inaddition to the magnitude of centrifugal force applied to the metalparts. When a siliceous film is too thick, crazing tends to occur in theformed siliceous film, thereby increasing the consumption of therust-inhibitive surface treatment agent solution to add the cost ofsurface treatment, and the siliceous film is preferably thinly formed inorder to satisfy the required rust-inhibitive performance.

Although the film thickness of a siliceous film formed on metal partsurfaces can be changed depending on the application, when the filmthickness is thinner than 0.5 μm, the film is difficult to impartrust-inhibitive performance for practical use. The siliceous film formedon the zinc surface preferably has an average thickness of 0.5 to 3 μmso that the rust-inhibitive performance for practical use can besatisfied. Crazing or peeling off hardly occurs in the siliceous filmhaving a thin average thickness, but the rust-inhibitive performance ispoor. On the contrary, when the siliceous film is too thick, crazing orpeeling off easily occurs, and therefore the average thickness is morepreferably 0.7 to 2 μm.

The film thickness of the coated film of the rust-inhibitive surfacetreatment agent formed on the metal part surface can be adjusted bychanging: the concentration of the alcoholic solution of thechromium-free rust-inhibitive surface treatment agent; the revolutionspeed of the spin coating method to control the centrifugal force; orthe added amount of a resin component having a thickening effect tocontrol the viscosity of the alcoholic solution of a chromium-freerust-inhibitive surface treatment agent.

The dip and spin coating method is suitable for applying arust-inhibitive surface treatment agent solution to metal parts withsmall sizes such as bolts and nuts. In addition to the dip and spincoating method, a dip drain coating method, a spray coating method,brush coating and the like can be employed depending on the size andshape of the product to apply an alcoholic solution of a chromium-freerust-inhibitive surface treatment agent.

The concentration of an alcoholic solution of a chromium-freerust-inhibitive surface treatment agent is preferably changed dependingon the method for coating metal parts, and when coated by the dip draincoating method or the spray coating method, the metal parts arepreferably coated with an alcoholic solution having a low concentration.

When only low-boiling point alcohol is used for a solvent of thechromium-free rust-inhibitive surface treatment agent, the solventrapidly evaporates when the room temperature is high, and theconcentration of the solution increases by the evaporation of alcohol,and therefore alcohol is replenished as required to maintain aconcentration suitable for coating.

Further, when a chromium-free rust-inhibitive surface treatment agent isapplied onto the surface of metal parts in a high humid room air ofrainy days, dew condensation occurs on the surface, and thereby thecoated film of the chromium-free rust-inhibitive surface treatment agentapplied may be degraded to reduce the rust-inhibitive performance of thesiliceous film.

In order to reduce the evaporation of alcohol and to suppress dewcondensation during summer season, 20 to 40 weight % of an alcoholcomponent in the alcoholic solution is preferably replaced with alcoholor glycol ether having a boiling point of 97° C. or more.

As the alcohol or the glycol ether having a boiling point of 97° C. ormore blended into the chromium-free rust-inhibitive surface treatmentagent solution, n-propyl alcohol, n-butyl alcohol, PGME, ethylene glycolmonoethyl ether and ETB can be used. When the humidity in the workingatmosphere where the rust-inhibitive surface treatment agent solution isapplied is high, dew condensation occurs on the surface of a formedfilm, thereby impairing the rust-inhibitive performance of the formedsiliceous film, but the ETB is a particularly preferred solvent havingan effect of preventing such an impairment of rust-inhibitiveperformance.

Coating of the chromium-free rust-inhibitive surface treatment agentfilm can be easily made on the zinc surfaces of a great number of metalparts having a small size at a time, and therefore it is preferred toapply the agent by the above dip and spin coating method.

It is preferred that the coated film of the rust-inhibitive surfacetreatment agent solution applied is dried and then baked while kept at atemperature of 120° C. or less to form a siliceous film. Since usersprefer low baking temperatures, the coated film can also be dried atroom temperature and then cured, but the rust-inhibitive performancewhich is imparted at 80° C. or less is a little poor. A siliceous filmis preferably formed by baking the coated film at 90 to 110° C. so thatthe siliceous film with good rust-inhibitive performance can beefficiently formed on the surface of the metal parts in a short periodof time. The heating time for baking the rust-inhibitive coated film at90 to 110° C. is preferably 10 to 25 minutes.

The zinc surface in the present invention may be an alloy containingzinc as a main component, and the chromium-free rust-inhibitive surfacetreatment agent of the present invention may be preferably applied ontovarious galvanized metal parts or die cast zinc parts.

EXAMPLES

Hereinafter, the present invention will be specifically described bymeans of examples, but should not be limited to these examples.

Synthesis of Alkoxysilane Oligomer

Into 25.5 parts by weight of titanium tetraisopropoxide (TA-10 producedby Matsumoto Fine Chemical Co., Ltd.) as a titanium alkoxide, were mixed60 parts by weight of isopropyl alcohol and 18 parts by weight ofacetylacetone to obtain a solution (exhibiting a yellow color) in whichabout a half of hydrolyzable isopropoxy groups contained in titaniumtetraisopropoxide are blocked with a chelate agent.

Next, into a raw material mixture solution obtained by mixing 250 partsby weight of ethyl silicate 40 (a product of Tama Chemicals Co., Ltd.,an oligomer of approximately pentamer obtained bycondensation-polymerizing tetraethoxysilane, and containing about 40weight % of silicon as converted to SiO₂), 25 parts by weight ofvinyltrimethoxysilane (SH6300 produced by Dow Corning Toray Co., Ltd.)and about 65 parts by weight of isopropyl alcohol, the above obtainedsolution in which about a half of isopropoxy groups of titaniumtetraisopropoxide are blocked with the chelate agent was mixed. Acidwater obtained by mixing 5.5 parts by weight of hydrochloric acidsolution (1 Normal) and 27.9 parts by weight of water was added to thissolution, and the resulting mixture was kept warm andcondensation-polymerized at 35° C. for 24 hours while stirring to obtainan alkoxysilane oligomer solution A shown in Column A of Table 1. 9.2mol % of the alkoxysilane raw materials used for the alkoxysilaneoligomer solution A (in this case, ethyl silicate 40 was deemed asequivalent monomers. Hereinafter, the same treatment shall be applied.)was an alkylalkoxysilane monomer.

In the alkoxysilane oligomer solution A obtained here, 4.7 atomic % ofsilicon was replaced with titanium and the total content of silicon andtitanium was 24.6 weight % when silicon and titanium were converted toSiO₂ and TiO₂, respectively. The weight-averaged molecular weight of thealkoxysilane oligomer was measured by a gel permeation chromatography(HLC-8120GPC manufactured by Tosoh Corporation) (using tetrahydrofuranas a solvent and polystyrene as standards) to give a value of 2010.

Similarly, the composition shown in Column B in Table 1 formulated bydecreasing the blending amount of vinyltrimethoxysilane was kept warmand condensation-polymerized at 35° C. for 24 hours while stirring toobtain an alkoxysilane oligomer solution B (the replacement ratio ofsilicon with titanium was 5 atomic % and the total content of siliconand titanium was 21.8 weight % when silicon and titanium were convertedto SiO₂ and TiO₂, respectively.). 2 mol % of the alkoxysilane rawmaterials used for the alkoxysilane oligomer solution B was analkylalkoxysilane monomer, and the quantity of the acetylacetone chelateagent was an amount of blocking about a half of isopropoxy groupscontained in the used titanium tetraisopropoxide. The alkoxysilaneoligomer had a weight averaged-molecular weight of 2270.

Next, the composition shown in Column C in Table 1 formulated byblending excess acetylacetone (in which acetylacetone corresponding to1.5 times isopropoxy groups contained in the used titaniumtetraisopropoxide was blended) was kept warm andcondensation-polymerized at 35° C. for 24 hours while stirring to obtaina yellow-colored alkoxysilane oligomer solution C (the replacement ratioof silicon with titanium was 6.6 atomic %, and the total content ofsilicon and titanium was 21.4 weight % when silicon and titanium wereconverted to SiO₂ and TiO₂, respectively.). 2 mol % of the alkoxysilaneraw materials used for the alkoxysilane oligomer solution C was analkylalkoxysilane monomer. The alkoxysilane oligomer had a weightaveraged-molecular weight of 1760.

Further, the composition shown in Column D of Table 1 which wasformulated using 16.7 mol % of γ-methacryloxypropyltrimethoxysilane(SH6030) as an alkylalkoxysilane monomer based on the total ofalkoxysilane raw materials and using n-butyl alcohol as a solvent inplace of isopropyl alcohol was kept warm and condensation-polymerized at35° C. for 24 hours while stirring to obtain an alkoxysilane oligomersolution D (the replacement ratio of silicon with titanium was 14.3atomic %, and the total content of silicon and titanium was 23.4 weight% when silicon and titanium were converted to SiO₂ and TiO₂,respectively.). It is to be noted that the amount of acetylacetone usedhere was an amount corresponding to 25% of isopropoxy groups containedin titanium tetraisopropoxide. The alkoxysilane oligomer had a weightaveraged-molecular weight of 1720.

Further, the composition shown in Column E of Table 1 which wasformulated using titanium tetra-n-butoxide in place of titaniumtetraisopropoxide was kept warm and condensation-polymerized at 35° C.for 24 hours while stirring to obtain an alkoxysilane oligomer solutionE (the replacement ratio of silicon with titanium was 12.2 atomic %, andthe total content of silicon and titanium was 18.7 weight % when siliconand titanium were converted to SiO₂ and TiO₂, respectively.). It is tobe noted that an alkylalkoxysilane monomer was not added to thealkoxysilane oligomer solution E. In addition, the amount ofacetylacetone used as a chelate agent was an amount corresponding to 33%of n-butoxy groups contained in titanium tetra-n-butoxide. Thealkoxysilane oligomer had a weight averaged-molecular weight of 1910.

Further, the composition shown in Column F of Table 1 which wasformulated using 8.0 mol % of methyltriethoxysilane (SZ6383 produced byDow Corning Toray Co., Ltd.) as an alkylalkoxysilane monomer based onthe total of alkoxysilane raw materials was kept warm andcondensation-polymerized at 35° C. for 24 hours while stirring to obtainan alkoxysilane oligomer solution F (the replacement ratio of siliconwith titanium was 4.9 atomic %, and the total content of silicon andtitanium was 22.7 weight % when silicon and titanium were converted toSiO₂ and TiO₂, respectively.). It is to be noted that the amount ofacetylacetone used here was an amount corresponding to 42% of isopropoxygroups contained in titanium isopropoxide. The alkoxysilane oligomer hada weight averaged-molecular weight of 1990.

Next, the composition shown in Column G in Table 1 which was formulatedwithout blending an alkylalkoxysilane monomer or a titanium alkoxide waskept warm and condensation-polymerized at 35° C. for 24 hours whilestirring to obtain an alkoxysilane oligomer solution G (the content ofsilicon was 20 weight % when silicon was converted to SiO₂.). Thealkoxysilane oligomer had a weight averaged-molecular weight of 2310.

Further, as shown in Column H of Table 1, the composition formulated byblending 2 mol % of vinyltrimethoxysilane in an alcoholic solution ofethyl silicate 40 based on the total of alkoxysilane raw materials andblending no titanium alkoxide was kept warm and condensation-polymerizedat 40° C. for 20 hours while stirring to obtain an alkoxysilane oligomersolution H (no titanium component was contained, and the content ofsilicon was 20.2 weight % when silicon was converted to SiO₂). Thealkoxysilane oligomer had a weight averaged-molecular weight of 2004.

Next, as shown in Column I of Table 1, the composition formulated byincreasing the blending amount of vinyltrimethoxysilane to 9.1 mol % ofthe total of alkoxysilane raw materials and blending no titaniumalkoxide was kept warm and condensation-polymerized at 40° C. for 20hours while stirring to obtain an alkoxysilane oligomer solution I (notitanium component was contained, and the content of silicon was 19.3weight % when silicon was converted to SiO₂). The alkoxysilane oligomerhad a weight averaged-molecular weight of 2020.

Further, as shown in Column J of Table 1, the composition formulated byblending vinyltrimethoxysilane, hydrochloric acid solution (1 Normal)and water to 250 parts by weight of ethyl silicate 40 was kept warm at35° C. for 24 hours to synthesize an alkoxysilane oligomer. After thealcoholic solution of the alkoxysilane oligomer synthesized was storedin a hermetically sealed state for about 2.5 years, the weightaveraged-molecular weight of the alkoxysilane oligomer in the alcoholicsolution was measured to be 7820. Since the weight averaged-molecularweight of the alkoxysilane oligomer immediately just after synthesizedwas usually about 2,000, it was understood thatcondensation-polymerization had further progressed during storage.

TABLE 1 Alkoxysilane Oligomer Solution (Composition in weight parts) A BC D E F G H I J Alkoxysilane Raw Material Tetraethoxysilane(Ethylsilicate 40) 250 250 250 200 225 240 250 250 250 250Alkylalkoxysilane Monomer Vinyltrimethoxysilane (SH6300) 25 5 5 5 25 5γ-methacryloxypropyl trimethoxysilane 66.6 (SH6030)Methyltriethoxysilane (SZ6383) 25 Titanium alkoxide Titaniumtetraisopropoxide (TA-10) 25.5 25.5 34.0 76.2 25.5 Titanium tetran-butoxide (TA-25) 70.8 Chelate agent Acetylacetone 18.0 18.0 72.0 26.827.3 15.0 1 Normal Hydrochloric acid 5.5 6.1 6.1 3.3 3.3 5.5 3.3 4.0 4.05.0 Water 27.9 24.4 24.4 26.0 23.4 30.0 26.0 27.0 27.0 27.0 Solventn-butyl alcohol 150 Isopropyl alcohol 125.0 171.1 130.8 220.0 151.5220.0 218.4 265.0 220.0 Total weight parts 476.9 500.1 522.3 548.9 569.8492.5 499.3 504.4 571.0 506.0 Replacement ratio (%) of silicon withtitanium 4.7 5.0 6.6 14.3 12.2 4.9 0.0 0.0 0.0 0.0 Alkylalkoxysilane(mol %) 9.2 2.0 2.0 16.7 0.0 8.0 0.0 2.0 9.1 2.0 Content (weight %)converted to SiO₂ + TiO₂ 24.6 21.8 21.4 23.4 18.7 22.7 20.0 20.2 19.319.8 Weight averaged molecular weight (Mw) 2010 2270 1760 1720 1910 19902310 2004 2020 7820 Ethylsilicate 40: Liquid of low molecular weightethoxysilane-oligomer available from Tama Chemicals Co., Ltd.(Containing pentamer molecules of tetraethoxysilane, as its mainconstituent, and Si of about 40 weight % as converted to SiO₂.)Alkoxysilane (mol %): Mol % of monomer converted alkoxysilane oligomer,and one mol of ethylsilicate 40 (pentamer, molecular weight: 744.5) iscounted to be 5 mole monomers. SH6300: Silane coupling agent(vinyltrimethoxy silane, molecular weight: 148.2) available from DowCorning Toray Co., Ltd. SH6030: Silane coupling agent(γ-methacryloxypropyl trimethoxysilane, molecular weight: 248) availablefrom Dow Corning Toray Co., Ltd. SZ6383: Methyltriethoxysilane(Molecular weight: 178) available from Dow Corning Toray Co., Ltd.TA-10: Titanium tetraisopropoxide (Molecular weight: 283.8) availablefrom Matsumoto Fine Chemical Co., Ltd. TA-25: Titanium tetra n-butoxide(Molecular weight: 339.9) available from Matsumoto Fine Chemical Co.,Ltd.

Example 1

Into 48 parts by weight of the alkoxysilane oligomer solution A, 7.5parts by weight of a 10 weight % ethyl cellosolve solution ofpolyvinylbutyral, 1 part by weight of a 0.6 weight % isopropyl alcoholsolution of boric acid and 44.5 parts by weight of isopropyl alcoholwere mixed to obtain the alcoholic solution of the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 1 shown in Table 2-1.The total content of silicon and titanium in the chromium-freerust-inhibitive surface treatment agent solution was 11.7 weight % whensilicon and titanium were converted to SiO₂ and TiO₂, respectively.

Next, five M8 bolts (half screws each with an underhead length of 45 mm)galvanized (plated thickness: 5 to 7 μm) in a zincate bath were placedand stirred in the chromium-free rust-inhibitive surface treatment agentsolution of EXAMPLE 1 in a container, and then taken out from thecontainer. The five bolts were placed in a stainless basket mounted on acentrifugal separator, and the basket was rotated at 700 RPM (radius ofgyration: about 150 mm) for 4 seconds to shake off an excess amount ofthe chromium-free rust-inhibitive surface treatment agent solutionadhered to the surfaces of the M8 bolts. Subsequently, the wet boltswith the rust-inhibitive surface treatment agent were put on a stainlesswire mesh, placed in a baking furnace and dried at 60° C. for 10minutes, and then the temperature was elevated to 100° C. and maintainedat 100° C. for 15 minutes to bake the bolts. One of M8 bolts coated withthe coated film of the chromium-free rust-inhibitive surface treatmentagent of EXAMPLE 1 was observed with a stereoscopic microscope(magnification: about 40 times) to examine the presence or absence ofcrazing. The other four M8 bolts were placed in a salt spray tester(SST) in accordance with JIS-Z-2371, and after 24 hours, one of thebolts was taken out, rinsed with water and dried, and then the surfaceof the bolt was observed with the stereoscopic microscope(magnification: about 40 times) to examine the presence or absence ofcrazing occurred in the rust-inhibitive coated film. The other threebolts were placed in the salt spray tester as they were, and thesurfaces of the bolts were observed with the naked eye every 24 hoursand the time when white rust or red rust was generated was recorded.

The time when white rust or red rust was generated shown in each ofTable 2-1, Table 2-2 and Table 3 is the time when white rust or red rustwas observed in two of the three bolts.

The bolt coated with a siliceous film of the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 1 hardly was crazedin the coated film to exhibit a good rust-inhibitive performance againstthe generation of white rust and red rust. Further, each of the boltsobserved with the stereoscopic microscope was buried in a resin to makea cut sample. The cross-section of the bolt in the cut sample wasobserved with the microscope to examine the film thickness of thesiliceous film, and the average film thickness observed was a littlethinner than 2 μm. In addition, the film thickness was also examined foreach of EXAMPLES 2 to 19 and Comparative EXAMPLES 1 to 3, and as aresult, all of the siliceous films coated on other bolts had an averagefilm thickness of 0.7 to 2 μm except for the siliceous film ofCOMPARATIVE EXAMPLE 1 which had an average film thickness of 2.3 μm.

It is a kind of acceleration test for causing crazing in a siliceousfilm that the M8 bolts coated with the siliceous film of thechromium-free rust-inhibitive surface treatment agent were put out fromthe salt spray tester after 24 hours, rinsed with water and then driedto examine the presence or absence of crazing occurred in the film witha stereoscopic microscope, and it is an alternative test to examinewhether crazing occurred in the rust-inhibitive film or not after the M8bolts coated with the siliceous film of the chromium-freerust-inhibitive surface treatment agent were left standing for a longperiod of time.

Example 2

Into 52.8 parts by weight of the alkoxysilane oligomer solution B, a 10weight % ethyl cellosolve solution of polyvinylbutyral, a 0.6 weight %isopropyl alcohol solution of boric acid and isopropyl alcohol weremixed to obtain the chromium-free rust-inhibitive surface treatmentagent solution of EXAMPLE 2 as shown in Table 2-1.

Example 3

Into 48.0 parts by weight of the alkoxysilane oligomer solution B, 3parts by weight of a 10 weight % ethyl cellosolve solution ofpolyvinylbutyral, 5 parts by weight of a 0.6 weight % isopropyl alcoholsolution of boric acid and 38 parts by weight of isopropyl alcohol weremixed to obtain the chromium-free rust-inhibitive surface treatmentagent of EXAMPLE 3.

Next, the chromium-free rust-inhibitive surface treatment agent of eachof EXAMPLE 2 and EXAMPLE 3 was applied onto five M8 bolts (half screwseach with an underhead length of 45 mm) by a dip and spin coating methodand then baked in the same manner as in EXAMPLE 1. The presence orabsence of crazing for one of the M8 bolts was examined with astereoscopic microscope, and the other four M8 bolts were placed in thesalt spray tester, and after 24 hours, one of the bolts was taken out,rinsed with water and dried, and then the presence or absence of crazingwas similarly examined with the stereoscopic microscope. The resultswere shown in Table 2-1. Further, the presence or absence of thegeneration of white rust or red rust for each of the three bolts placedin the salt spray tester was observed with the naked eye every 24 hours,and the time when the generation of white rust or red rust was observedin two of the three bolts was recorded in Table 2-1.

Both of the crazing resistance and the rust-inhibitive performance ineach of EXAMPLE 2 and EXAMPLE 3 were good, and the crazing observed inthe siliceous film coated on each of the M8 bolts which were placed inthe salt spray tester for 24 hours, taken out and then dried, wasslight, and it was a level causing no problem for practical use.

Examples 4 to 6

The chromium-free rust-inhibitive surface treatment agents of EXAMPLES 4to 6 were experimentally produced using the alkoxysilane oligomersolution B composed of the formulation compositions shown in Table 2-1.The chromium-free rust-inhibitive surface treatment agents of EXAMPLES 4to 6 were prepared by partly replacing easily evaporative isopropylalcohol having a low boiling point with PGME, ethyl cellosolve andn-propyl alcohol which are alcohol solvents having a high boiling point,respectively, in EXAMPLE 4 to EXAMPLE 6. Each of these chromium-freerust-inhibitive surface treatment agents was applied onto M8 bolts by adip and spin coating method, dried and then baked in the same manner asin EXAMPLE 1. The crazing resistance and the rust-inhibitive performancewere examined for these bolts in the same manner as in EXAMPLE 1. Allresults were good as shown in Table 2-1. Further, the evaporation ofalcohol from the alcoholic solution of the chromium-free rust-inhibitivesurface treatment agent could be made slow by blending an alcoholsolvent having a high boiling point, and even when the chromium-freerust-inhibitive surface treatment agent was applied onto metal partsduring the summer season when air temperature is high, the amount ofreplenishing alcohol evaporated to the rust-inhibitive surface treatmentagent could be decreased.

Example 7

The alkoxysilane oligomer solution C synthesized by excessively addingacetylacetone chelate agent was used to prepare the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 7 composed of theformulation composition shown in Table 2-1. The chromium-freerust-inhibitive surface treatment agent solution was applied onto fiveM8 bolts galvanized in a zincate bath by a dip and spin coating method,dried and then baked in the same manner as in EXAMPLE 1. The crazingresistance and the rust-inhibitive performance were examined for thesebolts in the same manner as in EXAMPLE 1, and all results were good asshown in Table 2-1, but the effect obtained by excessively addingacetylacetone could not be recognized.

Example 8

The alkoxysilane oligomer solution D synthesized by addingγ-methacryloxypropyltrimethoxysilane as alkylalkoxysilane monomer inplace of vinyltrimethoxysilane was used to prepare the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 8 composed of theformulation composition shown in Table 2-1. Next, this chromium-freerust-inhibitive surface treatment agent was applied onto five M8 boltsgalvanized in a zincate bath by a dip and spin coating method, dried andthen baked in the same manner as in EXAMPLE 1. The crazing resistanceand the rust-inhibitive performance were examined for these bolts in thesame manner as in EXAMPLE 1, and as a result, the rust-inhibitive coatedfilm became softened and the rust-inhibitive siliceous film causing nocrazing could be obtained, but the film resulted in a little poorrust-inhibitive performance, as shown in Table 2-1.

Example 9

The alkoxysilane oligomer solution E obtained by blending noalkylalkoxysilane monomer and using titanium tetra-n-butoxide as atitanium alkoxide was used to prepare the chromium-free rust-inhibitivesurface treatment agent of EXAMPLE 9 composed of the formulationcomposition shown in Table 2-1. Next, the chromium-free rust-inhibitivesurface treatment agent was applied onto five M8 bolts galvanized in azincate bath by a dip and spin coating method, dried and then bakedwhile maintained at 100° C. for 15 minutes in the same manner as inEXAMPLE 1. The crazing resistance and the rust-inhibitive performancewere examined for these bolts in the same manner as in EXAMPLE 1, andall results were substantially good as shown in Table 2-1.

Example 10

The alkoxysilane oligomer solution F which was condensation-polymerizedby adding methyltriethoxysilane and using titanium tetraisopropoxidechelated with acetylacetone was used, and PGME having a boiling point of121° C. was partly blended into alcohol in the alcoholic solution, andthereby the chromium-free rust-inhibitive surface treatment agent ofEXAMPLE 10 was prepared in the formulation composition shown in Table2-1. Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 15 minutes in the same manner as in EXAMPLE 11. The crazingresistance and the rust-inhibitive performance were examined for thesebolts in the same manner as in EXAMPLE 1, and all results weresubstantially good as shown in Table 2-1.

Example 11

Into 52.8 parts by weight of the alkoxysilane oligomer solution Hcontaining no titanium component which was condensation-polymerized byblending vinyltrimethoxysilane, 3 parts by weight of a 10 weight % ethylcellosolve solution of polyvinylbutyral, 5 parts by weight of a 1.2weight % isopropyl alcohol solution of boric acid, 23.8 parts by weightof isopropyl alcohol, 5.4 parts by weight of a titanium octylene glycolchelate compound produced by Nippon Soda Co., Ltd. (a compound whosealkoxy group is isopropoxide, hereinafter, referred to as “TOG”), 10parts by weight of PGME and 15 parts by weight of ETB were mixed toprepare the chromium-free rust-inhibitive surface treatment agent ofEXAMPLE 11 shown in Table 2-2. The total content of silicon and titaniumcontained in this chromium-free rust-inhibitive surface treatment agentsolution was 9.9 weight % when silicon and titanium were converted toSiO₂ and TiO₂, respectively, and the content ratio of titanium to thetotal amount of silicon and titanium was 4.6 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and all results were good as shown in Table 2-2.

Example 12

The alkoxysilane oligomer solution I containing no titanium componentwhich was condensation-polymerized by increasing the blending ratio ofvinyltrimethoxysilane was used to prepare the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 12 having theformulation composition shown in Table 2-2 in the same manner as inEXAMPLE 11. The total content of silicon and titanium in thischromium-free rust-inhibitive surface treatment agent solution was 9.5weight % when silicon and titanium were converted to SiO₂ and TiO₂,respectively, and the content ratio of titanium to the total amount ofsilicon and titanium was 4.8 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the rust-inhibitive performance was slightlyinferior to that of EXAMPLE 11, but all results were substantially goodas shown in Table 2-2.

Example 13

Using 52.8 parts by weight of the alkoxysilane oligomer solution H andblending the amount of TOG decreased to 3.2 parts by weight as comparedwith EXAMPLE 11, the chromium-free rust-inhibitive surface treatmentagent of EXAMPLE 13 having the formulation composition shown in Table2-2 was prepared. The total content of silicon and titanium in thechromium-free rust-inhibitive surface treatment agent solution was 11.0weight % when silicon and titanium were converted to SiO₂ and TiO₂,respectively, and the content ratio of titanium to the total amount ofsilicon and titanium was 2.7 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto the surfaces of the five M8 bolts galvanized in a zincatebath by a dip and spin coating method, dried and then baked whilemaintained at 100° C. for 20 minutes. The crazing resistance and therust-inhibitive performance were examined for these bolts in the samemanner as in EXAMPLE 1. As a result, by blending a smaller amount of thetitanium octylene glycol chelate compound than that of the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 11, therust-inhibitive performance was slightly inferior to that of EXAMPLE 11,but all results were substantially good as shown in Table 2-2.

Example 14

Into 52.8 parts by weight of the alkoxysilane oligomer solution H, 7.6parts by weight of a titanium octylene glycol chelate compound TC-200 (acompound produced by Matsumoto Fine Chemical Co., Ltd. whose alkoxygroup is n-octoxide, hereinafter referred to as “TC-200”) was blended toprepare the chromium-free rust-inhibitive surface treatment agent ofEXAMPLE 14 having the formulation composition shown in Table 2-2. Thetotal content of silicon and titanium in this chromium-freerust-inhibitive surface treatment agent solution was 11.3 weight % whensilicon and titanium were converted to SiO₂ and TiO₂, respectively, andthe content ratio of titanium to the total amount of silicon andtitanium was 4.6 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto the five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the rust-inhibitive performance was slightlyinferior to that of EXAMPLE 11, supposedly because alkoxide groups ofthe titanium octylene glycol chelate compound were different from thoseof EXAMPLE 11, but substantially good results were obtained as shown inTable 2-2.

Example 15

Into 52.8 parts by weight of the alkoxysilane oligomer solution H, atitanium chelate compound TC-100 (a product of Matsumoto Fine ChemicalCo., Ltd.) in which about a half of isopropoxy groups contained intitanium tetraisopropoxide were chelated by adding 2 mols ofacetylacetone to 1 mol of titanium tetraisopropoxide was blended toobtain the chromium-free rust-inhibitive surface treatment agent ofEXAMPLE 15 shown in Table 2-2. The total content of silicon and titaniumin the chromium-free rust-inhibitive surface treatment agent solutionwas 11.6 weight % when silicon and titanium were converted to SiO₂ andTiO₂, respectively, and the content ratio of titanium to the totalamount of silicon and titanium was 6.1 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto the surfaces of five M8 bolts galvanized in a zincate bathby a dip and spin coating method, dried and then baked while maintainedat 100° C. for 20 minutes. The crazing resistance and therust-inhibitive performance were examined for these bolts in the samemanner as in EXAMPLE 1, and as a result, the rust-inhibitive performancewas slightly inferior to that of EXAMPLE 11, but all results weresubstantially good as shown in Table 2-2.

Example 16

Together with TOG, both PGME and ethyl cellosolve that are high-boilingpoint alcohols were blended into 45.9 parts by weight of thealkoxysilane oligomer solution H to prepare the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 16 shown in Table2-2. The total content of silicon and titanium in the chromium-freerust-inhibitive surface treatment agent solution was 9.9 weight % whensilicon and titanium were converted to SiO₂ and TiO₂, respectively, andthe content ratio of titanium to the total amount of silicon andtitanium was 4.6 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the rust-inhibitive performance was slightlyinferior to that of EXAMPLE 11, but all results were substantially goodas shown in Table 2-2.

Example 17

Together with TOG, PGME and ethyl cellosolve that are high-boiling pointalcohols were blended into 52.8 parts by weight of the alkoxysilaneoligomer solution J to prepare the chromium-free rust-inhibitive surfacetreatment agent of EXAMPLE 17 shown in Table 2-2. The total content ofsilicon and titanium in the chromium-free rust-inhibitive surfacetreatment agent solution was 11.1 weight % when silicon and titaniumwere converted to SiO₂ and TiO₂, respectively, and the content ratio oftitanium to the total amount of silicon and titanium was 4.7 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, good results were obtained as shown in Table2-2.

Example 18

TOG was blended into 73.4 parts by weight of the alkoxysilane oligomersolution H to prepare the chromium-free rust-inhibitive surfacetreatment agent of EXAMPLE 18 shown in Table 2-2. The total content ofsilicon and titanium in the chromium-free rust-inhibitive surfacetreatment agent solution was 18.9 weight % when silicon and titaniumwere converted to SiO₂ and TiO₂, respectively, and the content ratio oftitanium to the total amount of silicon and titanium was 4.6 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the rust-inhibitive performance was slightlyinferior to that of EXAMPLE 11, but all results were substantially goodas shown in Table 2-2.

Example 19

Together with TOG, PGME and ETB that are high-boiling point alcoholswere blended into 45.9 parts by weight of the alkoxysilane oligomersolution H to prepare the chromium-free rust-inhibitive surfacetreatment agent of EXAMPLE 19 shown in Table 2-2. The total content ofsilicon and titanium in the chromium-free rust-inhibitive surfacetreatment agent solution was 6.6 weight % when silicon and titanium wereconverted to SiO₂ and TiO₂, respectively, and the content ratio oftitanium to the total amount of silicon and titanium was 4.6 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the rust-inhibitive performance was slightlyinferior to that of EXAMPLE 11, but both results were substantially goodas shown in Table 2-2.

Besides, the titanium chelate compound used in EXAMPLES 11 to 19 is acommercially available product in which a half of alkoxy groups in atitanium tetraalkoxide are blocked with a chelate agent.

TABLE 2-1 Components in Chromium-free Rust-inhibitive Surface TreatmentEXAMPLE (Composition in weight parts) Agent 1 2 3 4 5 6 7 8 9 10Alkoxysilane oligomer solution A B B B B B C D E F 48.0 52.8 48.0 52.852.8 52.8 52.8 48.0 48.0 65.0 Polyvinyl butyral resin solution 7.5 3.03.0 3.0 3.0 3.0 6.0 5.0 0.6 wt % boric acid solution 1 1 5 10 10 10 1Alcohol solvent Isopropyl alcohol 44.5 44.1 38.0 4.2 4.2 4.2 41.0 48.048.0 30.0 High-boiling point alcohol PGME 35.0 20.0 Ethyl Cellosolve35.0 n-propyl alcohol 35.0 Total weight parts 101.0 100.9 94.0 105.0105.0 105.0 100.8 96.0 96.0 120.0 Ti/(Ti + Si) (atomic %) 4.7 5.0 5.05.0 5.0 5.0 6.6 14.3 12.2 4.9 Content (wt %) converted to SiO₂ + 11.711.4 11.1 11.0 11.0 11.0 11.2 11.7 9.4 11.4 TiO₂ Appearance of crazesAfter coated and baked No No No No No No No No No No After 24 hours ofsalt spray No No A little bit A little bit A little bit A little bit Alittle bit No A little bit A little bit test Result of Salt spray testWhite rust (hours) 264 336 312 360 360 336 432 216 264 216 Red rust(hours) 1464 1656 1488 1680 1608 1584 1368 1224 1248 1200 Polyvinylbutyral resin solution: 10 weight % solution of polyvinyl butyral resin(BM-1: Available from Sekisui Chemical Co., Ltd.) solved in EthylCellosolve. 0.6 wt % boric acid solution: Isopropyl alcohol solution ofboric acid (0.6 wt %) PGME: Propylen glycol monomethyl ether EthyCellosolve: Ethylene glycol monoethyl ether

TABLE 2-2 Components in Chromium-free EXAMPLE (Composition in weightparts) Rust-inhibitive Surface Treatment 11 12 13 14 15 16 17 18 19Alkoxysilane oligomer solution H I H H H H J H H 52.8 52.8 52.8 52.852.8 45.9 52.8 73.4 45.9 Polyvinyl butyral resin solution 3 3 3 3 3 2.63 2.6 2.6 1.2 wt % boric acid solution 5 5 5 5 5 4.4 5 0 2 Alcoholsolvent Isopropyl alcohol 23.8 23.8 11 16.6 20 20.7 23.8 0 44High-boiling point alcohol PGME 10 10 15 15 15 8.7 10 0 16.7 EthylCellosolve 13 ETB 15 15 10 15 0 25 n-propyl alcohol Titanium Chelatecompound TOG 5.4 5.4 3.2 4.7 5.4 7.5 4.7 TC-200 7.6 TC-100 4.2 Totalweight parts 115 115 100 100 100 100 115 83.5 140.9 Ti/(Ti + Si) (atomic%) 4.6 4.8 2.7 4.6 6.1 4.6 4.7 4.6 4.6 Content (wt %) converted toSiO₂ + TiO₂ 9.9 9.5 11.0 11.3 11.5 9.9 11.1 18.9 6.6 Appearance ofcrazes After coated and baked No No No No No No No No No After 24 hoursof salt spray No No No No No No No A little bit No Result of Salt spraytest White rust (hours) 360 288 264 336 336 312 360 336 312 Red rust(hours) 1416 1152 1200 1320 1296 1224 1440 1320 1200 ETB: Ethylen glycolmonotertiary-butyl ether 1.2 wt % boric acid solution: Isopropyl alcoholsolution of boric acid (1.2 wt %) TOG: Titanium octyleneglycol chelatecompound available from Nippon Soda Co., Ltd. (Purity: 72%, Alkoxygroup: Isopropoxy group) TC-200: Titanium octylenglycol chelate compoundavailable from Matsumoto Fine Chemical Co., Ltd. (Purity: 67%, Alkoxygroup: n-octoxy group) TC-100: Chelated titanium compound formed byreacting 1 mol of titanium tetraisopropoxide with 2 mols ofacetylacetone. (Available from Matsumoto Fine Chemical Co., Ltd.)

Comparative Example 1

The alkoxysilane oligomer solution G in which silicon atoms are notreplaced with titanium atoms was used, and into this alkoxysilaneoligomer solution G, an ethyl cellosolve solution of a polyvinyl butyralresin, γ-glycidoxypropyltrimethoxysilane that is a silane coupling agenthaving an epoxy functional group, and nano-sized powder slurry oftitanium oxide (containing ethyl cellosolve and titanium oxide in aratio of 5:1) which was subjected to dispersion treatment by a bead millwere blended to prepare the chromium-free rust-inhibitive surfacetreatment agent of COMPARATIVE EXAMPLE 1 having the formulationcomposition shown in Table 3. The total content of silicon in thechromium-free rust-inhibitive surface treatment agent solution was 10.6weight % when silicon was converted to SiO₂. Next, the chromium-freerust-inhibitive surface treatment agent was applied onto the surfaces offive M8 bolts galvanized in a zincate bath by a dip and spin coatingmethod, dried and then baked while maintained at 100° C. for 15 minutesin the same manner as in EXAMPLE 1. The crazing resistance and therust-inhibitive performance were examined for these bolts in the samemanner as in EXAMPLE 1, and as a result, slight crazing was observed inthe rust-inhibitive coated film of the rust-inhibitive surface treatmentagent after baking, and the bolts were placed in a salt spray tester for24 hours, and then taken out and dried, and it was observed that crazingoccurred in the rust-inhibitive coated film on the surfaces of the driedbolts. In addition, as shown in Table 3, the rust-inhibitive performanceevaluated in a salt spray test was inferior to that of the bolt coatedwith each of the rust-inhibitive surface treatment agents of EXAMPLES 1to 19.

Comparative Example 2

The alkoxysilane oligomer solution G containing no titanium componentwas used, and the alkoxysilane oligomer solution G was diluted by addingisopropyl alcohol, and the chromium-free rust-inhibitive surfacetreatment agent of COMPARATIVE EXAMPLE 2 having the formulationcomposition shown in Table 3 was prepared. The total content of siliconin the chromium-free rust-inhibitive surface treatment agent solutionwas 10.8 weight % when silicon was converted to SiO₂.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, the results substantially similar to thoseof COMPARATIVE EXAMPLE 1 were obtained as shown in Table 3.

Comparative Example 3

Into 45.9 parts by weight of the alkoxysilane oligomer solution H, 8.8parts by weight of a zirconium chelate compound obtained by reacting 1mol of zirconium tetra-n-propoxide with 2 mols of 2-ethyl hexanoic acid(ZA-40) produced by Matsumoto Fine Chemical Co., Ltd., a 10 weight %ethyl cellosolve solution of polyvinylbutyral, a 0.6 weight % isopropylalcohol solution of boric acid, isopropyl alcohol, PGME and ETB weremixed to prepare the chromium-free rust-inhibitive surface treatmentagent of COMPARATIVE EXAMPLE 3 having the formulation composition shownin Table 3. The total content of silicon and zirconium in thechromium-free rust-inhibitive surface treatment agent solution was 11.2weight % when silicon and zirconium were converted to SiO₂ and ZrO₂,respectively, and the content ratio of zirconium to the total amount ofsilicon and zirconium was 8.9 atomic %.

Next, the chromium-free rust-inhibitive surface treatment agent wasapplied onto five M8 bolts galvanized in a zincate bath by a dip andspin coating method, dried and then baked while maintained at 100° C.for 20 minutes. The crazing resistance and the rust-inhibitiveperformance were examined for these bolts in the same manner as inEXAMPLE 1, and as a result, slight crazing occurred and therust-inhibitive performance was relatively good as shown in Table 3, butwas slightly inferior to that of the bolt coated with the chromium-freerust-inhibitive surface treatment agent of EXAMPLE 16 containing atitanium chelate compound blended therein. In addition, since zirconiumchelate compounds are expensive compounds as compared with titaniumchelate compounds, they are not practical for use due to high surfacetreatment cost.

TABLE 3 Components in Chromium-free COMPARATIVE EXAMPLE Rust-inhibitiveSurface (Composition in weight parts) Treatment Agent 1 2 3 Alkoxysilaneoligomer solution G G H 65.0 65.0 45.9 Polyvinyl butyral resin solution5.0 2.6 0.6 wt % boric acid solution 4.4 Alcohol solvent Isopropylalcohol 35 55 30.3 High-boiling point alcohol PGME 9 Ethyl Cellosolve 5ETB 14 Silane coupling agent γ-methacryloxypropyl 5.0 trimethoxysilane(SH6040) Zirconium Chelate compound 8.8 Titanium dioxide slurry 8.0Total weight parts 123 120 115 Zr/(Zr + Si) (atomic %) 8.9 Content (wt%) converted to SiO₂ + 10.6 10.8 11.2 ZrO₂ Appearance of crazes Aftercoated and baked A little A little No After 24 hours of salt spray testYes Yes A little bit Result of Salt spray test White rust (hours) 144120 192 Red rust (hours) 1080 1032 1104 ◯ Titanium dioxide slurry:Dispersion-treated slurry containing 16.7 weight % titanium dioxide finepowder dispersed in Ethyl Cellosolve. (Super titania F-6 available fromShowa Denko K.K. was used as titanium dioxide fine powder.) ◯ ZirconiumChelate compound: Formed by reacting of 1 mol of zirconium tetran-propoxide with 2 mols of 2-ethylhexanoic acid. The liquid containing30 weight % of zirconium is available from Matsumoto Fine Chemical Co.,Ltd.

1. A method for surface treating comprising: adding an acid catalyst andwater to an alcoholic solution containing alkoxysilane raw material tohydrolyze and condensation-polymerize the alkoxysilane raw material tosynthesize alkoxysilane oligomer having weight-averaged molecular weightof 1,000 to 10,000, mixing an organic chelate titanium compound with thealcoholic solution of the synthesized alkoxysilane oligomer to obtain achromium-free rust-inhibitive surface treatment agent, and surfacetreating a metal part with zinc surfaces with the chromium-freerust-inhibitive surface treatment agent, wherein silicon atoms inmolecules of the alkoxysilane oligomer are partly replaced with titaniumatoms from an organic chelate titanium compound, the alcoholic solutioncontains titanium of 2.5 to 15 atomic % to a total amount of silicon andtitanium, and the total amount of silicon and titanium is 5 to 20 weight% in the alcoholic solution when silicon and titanium are converted toSiO₂ and TiO₂, respectively.
 2. The method for surface treating as setforth in claim 1, wherein the organic chelate titanium compound istitanium alkoxide in which 40 to 60% of alkoxy groups are blocked orreplaced by a chelate agent.
 3. The method for surface treating as setforth in claim 1, wherein the alkoxysilane raw material consists of 90to 99 mol % of tetraalkoxysilane monomer or alkoxysilane oligomer, whicholigomer has weight-averaged molecular weight less than 800, and thebalance being alkylalkoxysilane monomer.
 4. The method for surfacetreating as set forth in claim 3, wherein the alkylalkoxysilane monomeris at least one selected from the group consisting ofmethyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane,vinyltrimethoxysilane and γ-methacryloxypropyltrimethoxysilane.
 5. Themethod for surface treating as set forth in claim 2, wherein the chelateagent is β-diketone or octyleneglycol.
 6. The method for surfacetreating as set forth in claim 1, wherein the alcoholic solution ofalkoxysilane oligomer contains 0.1 to 2 weight % of alcohol-solubleresin.
 7. The method for surface treating as set forth in claim 6,wherein the alcohol-soluble resin is polyvinyl butyral resin.
 8. Themethod for surface treating as set forth in claim 6, wherein thealcoholic solution of alkoxysilane oligomer contains 0.004 to 0.10weight % of boric acid.
 9. The method for surface treating as set forthin claim 1, wherein 20 to 40 weight % of alcohol component in thealcoholic solution of alkoxysilane oligomer is alcohol or glycol etherhaving a boiling point of 97° C. or more.
 10. The method for surfacetreating as set forth in claim 9, wherein the alcohol or the glycolether having a boiling point of 97° C. or more is at least one selectedfrom the group consisting of n-propyl alcohol, n-butyl alcohol,propylene glycol monomethyl ether, ethylene glycol monoethyl ether andethylene glycol monobutyl ether.
 11. A metal part with zinc surfacescoated with a siliceous film of 0.5 to 3 μm in average thicknessobtained by the method for surface treating as set forth in claim
 1. 12.A metal part with zinc surfaces as set forth in claim 11, wherein thesiliceous film is coated by a dip and spin coating method.
 13. A metalpart with zinc surfaces as set forth in claim 11, wherein the siliceousfilm is baked at a temperature of 120° C. or less.